There is evidence that the oxidative degradation of catechols, catecholamines and related compounds gives rise to toxic products: instances of phototoxicity of dopa derivatives in humans have been recorded, while the cytotoxicity and anti-tumor activity of several such compounds has been demonstrated, both in vitro and in vivo. ESR experiments are proposed to characterize and identify free radical species formed during the degradation of these materials and to elucidate the mechanisms of radical formation and decay. Specific compounds to be studied include dopa, the antitumor compounds dopa methylester, 3,4-dihydroxybenzylamine and 5,6-dihydroxyindole, 3,4-dihydroxyanilides, and the dopa metabolite 5-S-cysteinyldopa. Three modes of radical generation will be explored: photooxidation, autoxidation, and reduction of parent quinones with sulfyhydryl compounds. The mechanism of photooxidation of dopa and related compounds will be studied and compared with that of tyrosine; action spectra and quantum yields will be measured. Evidence for excited state precursors will be sought making use of excited state quenchers and the chemically induced dynamic electron polarization (CIDEP) phenomenon. Autoxidation gives oxygen radicals and o-quinones which can react with sulfhydryl groups in sensitive enzymes. The importance of radical production from o-quinones and sulfydryl compounds will be assessed.